Implosion of thin wall stainless steel tanks in the wine industry has been very prevalent and costly and over the years many attempts have been made to solve the problem of providing a seal which is readily broken upon rapid drop of pressure within the tank, due for example to rinsing with cold water after a hot caustic wash, or by egress of contents of a wine tank by pumping out wine, and which will allow air to replace the space previously occupied by the liquid. Because of the failure of prior art tank lids to satisfy all requirements, implosions still occur regularly and the cost of a large wine tank is very high. Overpressurisation can occur due to many operational reasons and can also seriously damage tanks. One of these reasons is the reaction of ferments which produces voluminous amounts of carbon dioxide gas as well as expanding the liquid contents by heating, the reaction being exothermic.
A commercial pressure-vacuum valve is known to exist, comprising an annular membrane which depends from an annular frame, and seals around a valve seat when there is no pressure differential between the interior of a tank and surrounding atmosphere, or when there is pressure in the tank, but lifts away from the valve seat under vacuum conditions within the tank. Upon pressure conditions occurring within the tank, a pressure valve carried by the annular frame will lift to allow egress of air from the tank. There are several reasons given by wine makers for not widely adopting use of such valves, including cost, size, complexity, and fear of adhesion between the membrane and valve seat, which is a machined seat.
The most successful attempt which has been made heretofore to avoid implosion of a wine tank and is known to the applicant is embodied in applicant's U.S. Pat. No. 4736864 for a tank lid of a different type from that disclosed in this invention, but although tank lids according to that invention have been reasonably successful there is still room for improvement in certain aspects. Particularly the lid must allow free ingress of air when vacuum occurs inside the tank, such as to prevent vacuum in the tank exceeding 50 mm of water gauge at any flow rate, and flow rate can be as much as 0.5 m.sup.3 per second in a 100,000 liter tank. Secondly, it is desirable that the lid should last for many years without deterioration in its operation and should avoid having any finish machined surfaces because of cost, nor any adjustable items that could require periodical resetting. Further, if used for wine the tank neck lip will normally become contaminated with sticky wine residue, and the tank lid must be capable of maintaining pressure/vacuum specifications by overcoming the adhesive effects of the wine residue. Still further, it is desirable that the tank lid should be operable to allow a tank washing spray ball to pass through the tank lid and at that time to maintain its pressure/vacuum control phases.
Still further, the lid should be capable of alternating between pressure and vacuum modes without attention from any person. These are only five of twenty desiderata which have been identified and may be regarded as requirements for a tank lid, although they include the five most difficult to achieve.
BRIEF SUMMARY OF THE INVENTION
The main object of this invention therefore is to provide a tank lid which will function to achieve all the abovementioned objects as well as other well known requirements of a tank lid, and in an embodiment a tank lid has a membrane of sufficient dimension to overlap the peripheral edge of a tank neck, an aperture in the membrane which opens to a space beneath an imperforate membrane support supporting the periphery of the membrane, and brackets arranged to be secured to a tank neck to be outstanding therefrom, and in use, to carry the membrane support when the lid is in a (normal) closed position.
Since the membrane support is itself imperforate, under normal operating conditions the same pressure will exist in the space between the membrane and support as exists in the tank. Under vacuum conditions however a low pressure will be drawn above the membrane and the higher atmospheric pressure acting on the membrane annulus where it overlaps the tank neck will initially lift the membrane away from the lip of the neck, allowing ingress of air the flow of which will retain the membrane lifted. The third mode is when the pressure within the tank increases and that pressure operates so as to lift the membrane and all the elements of the lid from the lip of the tank neck.
More specifically, the invention consists of a tank lid suitable for sealing a tank around a lip which defines an opening in a neck of the tank, comprising a rigid imperforate membrane support, a membrane of dimension exceeding the dimension of the opening, means sealably securing the periphery of the membrane to the membrane support, an aperture in the membrane opening to space between the membrane and said support, and brackets arranged to be outstanding from the neck to carry the membrane support when the lid is in a normal (closed) position.